This invention relates to liquid crystal displays (LCD), and more particularly, to temperature stabilization of LCDs. More specifically, the present invention deals with temperature stabilization of an LCD of the type used in devices such as cellular telephones.
Liquid crystal displays (LCD) are commonly used in a number of diverse products to show the operational status of the product to a user of the product. These products may typically be hand-held, battery operated devices such as personal digital assistants, scanners and cellular telephones. The performance of LCDs in these products deteriorate when the product is used in low ambient temperatures such that the display of alphanumeric characters and graphics becomes slower as the ambient temperature decreases.
The deterioration of LCD performance in low ambient temperatures is particularly troublesome to both cellular telephone users and cellular telephone performance. A particularly troublesome aspect encountered by a cellular telephone user is the slow response of the LCD as the user attempts to access the cellular telephone features or process a cellular communication using a cellular telephone that has been exposed to subzero or low ambient temperatures. Slow response of the LCD in a cellular telephone is not acceptable and due to battery power consumption constraints, it is not possible to use battery power to warm the LCD sufficiently to assure reliable performance in low ambient temperatures.
A further troublesome aspect is the additional battery power that is consumed as the cellular telephone user waits for the LCD display to become operational thus leading to a shorter battery life. Cellular telephones are marketed as xe2x80x9cstate of the artxe2x80x9d communication devices and must maintain functionality in all typical user environments, particularly out-of-doors if these devices are to maintain their marketing images as advanced xe2x80x9cstate of the artxe2x80x9d products.
A further problem which is manifested by unreliable or poor performance of a product is due to excessive heat developed by electrical components in the product. This excessive heat can cause expansion and contraction which effects the electrical and/or mechanical connection of the component to the printed circuit board. The excessive heat can also degrade the performance and reliability of the component itself which in turn results in less than acceptable product operation. Cellular telephones are susceptible to such reliability problems resulting from excessive heat developed by the phone circuitry and particularly excessive heat developed in the RF component section.
It is well known to those in the art that cellular telephone circuitry components, particularly RF components in the output stage generate and radiate excessive thermal energy or heat due to the components inefficiencies. Typically, the transmit efficiency in a GSM (global system for mobile communications) cellular telephone is in the range of 30 to 40 percent (%). For illustrative purposes to estimate the power dissipation converted to thermal energy, we will take that the maximum transmit power of a GSM cellular telephone is 2 Watts and the pulse duty cycle is xe2x85x9. The average transmit power is then 0.25 Watts. The power dissipation converted into thermal energy is 2 times 0.25 Watts or 0.5 Watts. In reality the actual power dissipation is somewhat less due to pauses in speech and typically is in the range of 0.1 to 0.2 Watts. Two immediate benefits are achieved by leading the wasted or excess thermal energy away from the RF components. First, the operating temperature of the RF components will be lower thus making the RF components more stable, and second, by directing the wasted thermal energy through the LCD, its operational temperature will, in contrast to the RF components, be higher thus shortening the response time of the LCD.
Accordingly, it is an object of the present invention to provide a passive means and method for temperature stabilization for liquid crystal displays (LCD).
It is a further object of the present invention to dissipate excessive thermal energy or heat generated by electrical circuit components in a cellular telephone, particularly excessive heat generated in the RF component section to lower the operating temperature of the RF components.
It is yet a further object of the present invention to direct the excessive heat generated by the RF components to warm the LCD to shorten its response time in low ambient temperatures.
According to the present invention, a temperature stabilizer for an LCD device absorbs excess thermal energy or heat from an electrical circuit component wherein a thermally conductive medium guides the absorbed heat for transfer to the surface of the LCD.
In an embodiment shown herein, an electromagnetic shielding enclosure covers the electrical circuit components and absorbs thermal energy or heat generated by and radiated from the electrical circuit component. An LCD device is positioned above the enclosure and the absorbed heat is transferred by air convection to warm the LCD.
In a further embodiment as shown herein, the electromagnetic shielding enclosure includes an integrally coupled thermally conductive guide member to conduct the absorbed heat to the LCD which may be positioned other than directly above the enclosure.
According to a further embodiment of the invention shown herein, the electromagnetic shielding enclosure covers the RF component section in a cellular telephone to absorb the excess thermal energy or heat generated by and radiated from the RF component resulting in the RF component operating at a lower temperature. The absorbed heat is conducted along a thermally conductive guide member to warm the LCD which may be in physical contact with the guide member and positioned other than directly above the enclosure.